A method of this type is known from German Pat. No. 26 45 736.
In this patent, to calibrate a transducer using a suitable calibration solution, such as water or sodium sulfite, an overlay is provided which is adapted to the measuring surface of the transducer. The overlay has a calibration volume, which must be positioned exactly over the measuring surface. The calibration solution is introduced into this calibration volume, retained there and then removed once again. The overlay comprises an adhesive layer, for example, and a covering layer disposed over it; a felt-like inlay may be provided in the adhesive layer, which by suitable means is disposed centrally over the measuring surface in order to receive the calibration solution.
Typically, two different calibration solutions are selected for calibrating the known transducers; where a transducer for transcutaneous determination of the oxygen content of blood is to be calibrated, these solutions are pure water and a solution of sodium sulfite in liquid or paste form. In both cases, care must be taken that the appropriate calibration volumes be disposed exactly above the measuring surface, using localizing rings provided to that end. Air bubbles must be prevented from forming in the calibration solutions introduced into the calibrating volumes, and the calibration volumes must cover the measuring surface completely during the entire calibration procedure. Because of these requirements, the transducer must be supported in a prescribed way during the calibration procedure. Transducers for transcutaneous determination of the blood oxygen content, for example, can accordingly be calibrated by the known method only in a horizontal position.
Preparing the calibration solutions also necessitates further handling steps, which must be performed before the calibration itself is done. Specifically, the required calibration solutions must first be mixed according to specific prescriptions, and the shelf life of the mixed solutions is limited. To prepare a suitable sodium sulfite solution, for instance, first a specific amount of sodium sulfite must be dissolved in pure water, and then it must be introduced drop by drop into the calibrating volume, using additional equipment such as pipettes. Once the sodium sulfite solution has been prepared, it cannot be held for a relatively long period; instead, typically after a period of a month, it must be discarded and prepared anew.
Once calibration has been performed, both the calibration solution and the retaining rings used to center the calibration volume must be removed, and any residues of the calibration solution must be particularly carefully removed from the measuring surface.
European Pat. application No. EP-A-31 149 does teach that the usability of calibration solutions, in particular sodium sulfite, can be indicated from the shift in the pH value by means of a suitable color indicator, and that the calibration solution can be discarded as unusable after a certain threshold pH value is attained, as determined by the color change, for instance from blue to yellow. However, this does not spare the user the work of mixing a new solution, which again has only limited shelf life, once a given calibration solution has become unusable.
Furthermore, if even slight amounts of weak acids or alkalis are present in the calibration solution due to oversight, the result is an uncontrolled shift in the pH value, which means that a direct correlation between a color change and the usability or nonusability of the calibration solution can no longer be made.